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A long look by NASA's Chandra X-ray Observatory has revealed new evidence that extremely hot gas exists in a large region at the center of the Milky Way. The intensity and spectrum of the high-energy X-rays produced by this gas present a puzzle as to how it is being heated.

This image was produced by combining a dozen Chandra observations made of a 130 light year region in the center of the Milky Way. The colors represent low (red), medium (green) and high (blue) energy X-rays. Thanks to Chandra's unique resolving power, astronomers have now been able to identify thousands of point-like X-ray sources due to neutron stars, black holes, white dwarfs, foreground stars, and background galaxies. Credit: NASA/CXC/UCLA/MIT/M.Muno et al.

The discovery came to light as a team of astronomers, led by Michael
Muno of UCLA used Chandra's unique resolving power to study a region
about 100 light years across and painstakingly remove the contributions
from 2,357 point-like X-ray sources due to neutron stars, black holes,
white dwarfs, foreground stars, and background galaxies.

What remained was an irregular, diffuse glow from a 10-million-degree
Celsius gas cloud, embedded in a glow of higher-energy X-rays with a
spectrum characteristic of 100-million-degree gas.

"The best explanation for the Chandra data is that the high-energy
X-rays come from an extremely hot gas cloud," says Muno, lead author on
a paper describing the results to appear in the September 20, 2004 issue
of The Astrophysical Journal. "This would mean that there is a
significant shortcoming in our understanding of heat sources in the
center of our Galaxy."

The combined gravity from the known objects in the center of the Milky
Way -- all the stars and the supermassive black hole in the center -- is
not strong enough to prevent the escape of the 100 million degree gas
from the region. The escape time would be about 10,000 years, a small
fraction of the 10-billion-year lifetime of the Galaxy. This implies
that the gas would have to be continually regenerated and heated.

The gas could be replenished by winds from massive stars, but the source
of the heating remains a puzzle. The high-energy diffuse X-rays from the
center of the Galaxy appear to be the brightest part of a ridge of X-ray
emission observed by Chandra and previous X-ray observatories to extend
for several thousand light years along the disk of the Galaxy. The
extent of this hot ridge implies that it is probably not being heated by
the supermassive black hole at the center of the Milky Way.

Scientists have speculated that magnetic turbulence produced by
supernova shock waves can heat the gas to 100 million degrees.
Alternatively, high-energy protons and electrons produced by supernova
shock waves could be the heat source. However, both these possibilities
have problems. The spectrum is not consistent with heating by
high-energy particles, the observed magnetic field in the Galactic
center does not have the proper structure, and the rate of supernova
explosions does not appear to be frequent enough to provide the
necessary heating.

The team also considered whether the high-energy X-rays only appear to
be diffuse, and are in fact due to the combined glow of an as yet
undetected population of point-like sources, like the diffuse lights of
a city seen at a great distance. The difficulty with this explanation is
that 200,000 sources would be required in the observed region. Although
the total number of stars in this region is about 30 million, the number
of stars of the type expected to produce X-rays at the required power
and energy is estimated to be only 20 thousand. Further, such a large
unresolved population of sources would produce a much smoother X-ray
glow than is observed.

"There is no known class of objects that could account for such a large
number of high-energy X-ray sources at the Galactic center," said Fred
Baganoff of the Massachusetts Institute of Technology (MIT) in
Cambridge, a coauthor of the study.

These results were based on over 170 hours of observations of a
17-by-17-arcminute region around the Milky Way's center using Chandra's
Advanced CCD Imaging Spectrometer instrument. Other team members from
UCLA, MIT, and Penn State are also co-authors on the upcoming paper in
The Astrophysical Journal.